4. I N S ITU X-R AY A BSORPTION S PECTROSCOPY S TUDY ON O XYGEN R EDUCTION AND
4.2. E XPERIMENTAL M ETHODS
4.2.1. Synthesis of Size-Controlled CoOx NPs
CoOx NPs were synthesized as described in a previously report with some modifications.26 Standard Schlenk techniques were used, and all manipulations with the cobalt carbonyl precursor were performed in a glove box. First, 73 μL of oleic acid (99%, Sigma-Aldrich) was added in a 100 mL round bottom flask, evacuated for 10 min, and saturated with Ar. Then, 7.5 mL of anhydrous o- dichlorobenzene (o-DCB, 99%, Sigma-Aldrich) were added. The flask was equipped with a Liebig condenser, volume spacer, and release line to accommodate the large volume of CO, which was produced upon decomposition of the carbonyl precursor. With vigorous stirring, the mixture was heated to a desired temperature (164, 168, 176, or 182 °C) from RT at a heating rate of 5 °C min−1 under an Ar atmosphere. After temperature stabilization, 1.5 mL of 0.5 M Co2(CO)8 (Sigma-Aldrich) dissolved in o-DCB were quickly injected into the hot solution. The transparent and brownish solution immediately turned black, indicating the formation of NPs. This colloidal suspension was aged for 20 min and then cooled in a flow of air. To separate the CoOx NPs 5 mL of o-DCB and 25 mL of 2- propanol (99%, Sigma-Aldrich) were added to the suspension, followed by centrifugation at 8000 rpm for 15 min. The supernatant was decanted, and the precipitate was dispersed in chloroform (99.5%,
Samchun chemical).
4.2.2. Preparation of CoOx/CNTs
Before the CoOx NPs were supported onto the CNTs, as-received CNTs were acid-treated to remove metallic impurities and to functionalize the CNT for better adhesion of the NPs. 2.5 g of the pristine multi-walled CNTs (MR 99, Carbon Nano-material Technology Co.) was mixed with 380 g of 6 M HCl (diluted from 35–37% HCl, Samchun chemical), and the mixture was stirred at 80 °C for 12 h. The suspension was filtered, washed with a large amount of DI water until the pH of the filtrate reached ~7, and dried at 60 °C. The HCl-treated CNTs were subsequently treated with 390 g of 6 M HNO3 (diluted from 60% HNO3, Samchun chemical) in the same manner. Then, the CoOx NPs were supported on CNTs as follows. First, 350 mg of the acid-treated CNTs were dispersed in 50 mL of chloroform in a 100 mL Erlenmeyer flask. After stirring for 15 min in the closed flask, 38.9 mg of the as-prepared CoOx NPs dispersed in chloroform (corresponding to the target loading of 10 wt% of CoOx), was added dropwise to the solution. Subsequent sonication in ice-cold water for 3 h led to the homogeneous dispersion of CoOx NPs on CNTs. The product was separated by centrifugation and decantation, and dried at 60 °C. Finally, the surfactants (i.e., oleic acid) surrounding the NPs were removed following a previously reported method.30 The dried CoOx/CNTs was annealed at 185 °C for 5 h under air (temperature ramping rate: 1.4 °C min−1). For fair comparison, the acid-treated CNTs without CoOx were also annealed and used for further characterizations.
4.2.3. Synthesis of Bulk-CoOOH
Bulk-CoOOH was synthesized for the use as a reference material for the XAS.31 First, Co(OH)2
powder (95%, Sigma-Aldrich) was added to 40 mL of DI water. 10 mL of 8 M NaOH (diluted from 98% NaOH, Samchun chemical) was added dropwise, and subsequently 4 mL of H2O2 (30%, Sigma- Aldrich) was added at once with vigorous stirring. This reaction explosively produces O2 gas. The mixture was stirred at 45 °C for 28 h. The suspension was filtered, washed with DI water several times, and dried at 60 °C. The resulting CoOOH was found to be phase-pure with large crystallite size as revealed by XRD (Figure 4.1)
Figure 4.1. XRD pattern of the synthesized bulk-CoOOH and a standard CoOOH.
4.2.4. Characterization Methods
High-resolution transmission electron microscopy (HR-TEM) images were taken on a JEOL JEM-2100 electron microscope at an acceleration voltage of 200 kV. Atomic-resolution TEM (AR- TEM) images were taken with a low-voltage spherical aberration-corrected TEM (FEI Titan3 G2 60- 300 with an image Cs corrector) with an acceleration voltage of 80 kV. X-ray powder diffraction (XRD) patterns were obtained with a high-power X-ray diffractometer (D/MAX2500/PC, Rigaku) equipped with Cu Kα radiation, and operated at 40 kV and 200 mA. Wide-angle XRD patterns were measured in a 2θ range from 10° to 80° at a scan rate of 4° min−1. The Co content in the catalysts was determined using an inductively coupled plasma optical emission spectrometry (ICP-OES) analyzer (700-ES, Varian). The ICP-OES analysis results are summarized in Table 4.1.
Table 4.1. Co contents in the CoOx/CNTs analyzed by ICP-OES.
Sample Co contents (%) CoOx(4.3)/CNTs 11.9 CoOx(6.3)/CNTs 11.5 CoOx(7.5)/CNTs 12.1 CoOx(9.5)/CNTs 12.6
4.2.5. XAS Experiments
XAS experiments were conducted on the beamlines 6D and 10C of the Pohang Accelerator Laboratory (PAL) in South Korea with a beam energy and current of 3 GeV and 300 mA, respectively.
X-ray photon energy was filtered with a Si(1 1 1) double-crystal monochromator, which was detuned by around 15% and 30% at the 6D and 10C beamlines, respectively, to remove high-order harmonics.
In situ XAS spectra were obtained by using a home-made spectroelectrochemical cell in fluorescence mode (Figure 4.2). Catalyst ink (described in Section 4.2.6) was deposited and dried on a piece of carbon fiber paper. The catalyst film was attached on the window of the cell using a Kapton tape, while the catalyst layer was facing inward of the cell to be contacted with the electrolyte (0.1 M KOH). In situ XAS measurement was firstly conducted at the open circuit voltage (OCV), and the subsequent XAS scan was performed after applying ORR (0.6 V vs RHE, iR-corrected) or OER (1.8 V) potential for 1 h in order to give enough time for phase transformation. Background removal and normalization of the spectra were carried out by using IFEFFIT (Athena) software.32
Figure 4.2. Schematic illustration and photograph of the home-made spectroelectrochemical cell and experimental setup.
4.2.6. Electrochemical Characterizations
Electrochemical characterizations of the catalysts were performed using an IviumStat electrochemical analyzer at RT and atmospheric pressure, using a three-electrode system. A graphite counter electrode and an Hg/HgO reference electrode (1 M KOH filling solution) were used. All potentials in this report were converted to the reversible hydrogen electrode (RHE) scale (experimental details in Section 3.2.5).
A rotating ring-disk electrode (RRDE, ALS) comprised of a glassy carbon (GC) disk (4 mm in diameter) and a Pt ring was used as a working electrode. The RRDE was polished with a 1.0 μm alumina suspension and then with a 0.3 μm suspension to generate a mirror finish before every use.
The catalyst ink was prepared by mixing catalyst (7.5 mg), neutralized Nafion (0.2 mL), DI water (0.1 mL), and absolute ethanol (0.9 mL) and by sonicating for at least 1 h. Neutralized Nafion was prepared by mixing 0.1 M NaOH (diluted from 99.99% NaOH, Sigma-Aldrich) and Nafion (5 wt%, Sigma-Aldrich) in a ratio of 1:2 (v:v), considering the proton concentration of Nafion (~0.05 M), to minimize any transformation of the catalyst during the ink preparation.33 Next, 3 μL of the catalyst ink were pipetted with a micro-syringe (Hamilton) and deposited onto the GC electrode and dried at 70 °C for 2 min. The catalyst loading was 0.15 mg cm−2.
To investigate the redox properties of the samples, cyclic voltammetry (CV) from 0.05 to 1.50 V (vs RHE) was conducted in N2-saturated 1 M KOH at a scan rate of 20 mV s−1. Before the activity measurement, electrochemical impedance spectroscopy (EIS) was performed around at the OCV with a potential amplitude of 10 mV from 10000 to 1 Hz. Series resistance was determined at a high frequency intercept on the x-axis (real part of the impedance) of the EIS spectra, which was used to correct the iR-drop. The OER activity was obtained from 10 scans of CV in the range of 1.2 to 1.8 V (vs RHE) at a scan rate of 20 mV s−1 with an electrode rotation at 1,600 rpm to efficiently remove evolved O2. The cathodic and anodic currents of the 10th CV were averaged. Linear sweep voltammetry (LSV) was performed to obtain the ORR polarization curves by sweeping the potential from 1.1 to 0.2 V (vs RHE) at a scan rate of 5 mV s−1 in O2-saturated 0.1 M KOH with O2 bubbling at a rotating speed of 1,600 rpm. The OER/ORR measurements were independently repeated three times, and the averaged and iR-compensated (100%) data are presented.
For the evaluation of the kinetics for the ORR, the kinetic current was extracted from the following equation
d
k j
j j
1 1
1 = +
where j, jk, and jl represent the measured current density, the kinetic current density, and diffusion- limited current density, respectively (normalized by GC electrode area).
The logarithmic plot of the kinetic current density (the measured current density in the case of the OER) versus the overpotential gives a linear Tafel plot
log 0
log j b j
b
η= - k +
where η, b, and j0 indicate the applied overpotential, the Tafel slope, and the exchange current density,
respectively.
Four-electron selectivity of the CoOx/CNTs was analyzed by RRDE technique and calculated using a given equation
d r
i N n i
+ ´
= 1
4
where n, N, ir, and id stand for the electron transfer number (selectivity), the collection efficiency (0.40, provided by the manufacturer) indicate the applied overpotential, the Tafel slope, and the exchange current,